A conventional vertical planetary gear reducer for a mixer usually includes a fixed ring gear and a rotatable planetary carrier, so that power is output from an output shaft mounted on the planetary carrier. The planetary gear reducer with the above structure requires an oil seal mounted on the output shaft to prevent an oil leak of the planetary gear reducer. When the planetary gear reducer has been used over a long period of time, the material forming the oil seal tends to become aged and a position on the output shaft in contact with the oil seal will be worn out to cause an oil leak. The leaked oil will contaminate materials being mixed.
When the conventional planetary gear reducer is used with a stand mixer, the oil leaked from the reducer will seriously contaminate the products being mixed and accordingly results in big losses to the manufacturers of the mixed products. Even though some reliable and relatively expensive mechanical oil seals (e.g. floating seals) have been used in some advanced planetary gear reducers, the condition of oil leak still exists and could not be thoroughly overcome. In the case the materials to be mixed include food or medicine, the oil leak of the planetary gear reducer will cause even more serious adverse results.
FIG. 1 is a schematic structural view of a conventional vertical planetary gear reducer, which includes a motor 10, a gearbox 11, a coupling 12, and an output shaft 13. The gearbox 11 has a planetary gear reducing mechanism (not shown) received therein. The coupling 12 couples a power output shaft 14 of the gearbox 11 to the output shaft 13, so that power is output via the output shaft 13. For the output shaft 13 to have good radial pressure resistance, two bearings 15, 16 mounted around the output shaft 13 should not be located too close to each other. Therefore, a support base 17 for supporting the two bearings 15, 16 must also be long enough to cover the long distance between the two bearings 15, 16. With this structure, the whole planetary gear reducer will have a relatively large overall length, and an oil seal 18 mounted on the output shaft 13 tends to be worn out and cause an oil leak when the output shaft 13 has been used over a long period of time.